A honeycomb structure is mounted as a catalyst carrier in a catalytic converter used for treating harmful components [hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx)] contained in an exhaust gas emitted from gasoline engine. A large amount of particulate matter composed mainly of soot (black smoke of carbon) is contained in an exhaust gas emitted from diesel engine, gasoline lean-burn engine or direct injection gasoline engine; discharge of the particulate matter into the air induces environmental pollution; therefore, a honeycomb filter is mounted in the exhaust system of diesel engine for capturing of the particulate matter. A honeycomb structure is used in such a honeycomb filter as well.
The honeycomb structure used for such purposes comprises a large number of through-holes (cells) defined by porous partition walls and extending in the axial direction of honeycomb structure. An exhaust gas passes through the cells and is treated by the catalyst component supported on the partition walls defining the cells. As the honeycomb structure, there is generally used a cordierite-based ceramic honeycomb structure or silicon carbide-based ceramic honeycomb structure which is formed by extrusion and supplied inexpensively in a large amount. Or, there is used a metal-made honeycomb structure obtained by winding a thin flat plate and a wavy plate alternately into a corrugated shape (see, for example, Patent Literature 1 or 2).
As the form of the filter, there is mentioned a form wherein the ends of given cells are plugged at one end face of honeycomb structure and the ends of residual cells are plugged at other end face, or a form wherein plugging is made only at one end face. A fluid such as an exhaust gas enters those cells which are not plugged at the inlet side end face of filter and are plugged at the outlet side end face, passes through the porous partition walls, moves into those cells which are plugged at the inlet side end face and are not plugged at the outlet side end face, and is discharged. At this time, the partition walls function as a filter layer, and the particulate matter such as soot contained in the exhaust gas is captured by the partition walls and deposits thereon.
In conventional honeycomb structures, the cells have had about the same size at all cell sections normal to their lengthwise direction (the flow direction of fluid) and, accordingly, the partition walls defining adjacent cells have been approximately parallel. Also, the outer diameter of honeycomb structure has been generally about the same over the entire lengthwise direction (the flow direction of fluid). Unlike the honeycomb structure having such a shape, there is also proposed a honeycomb structure wherein the size of cell section is gradually changed over the entire length of honeycomb structure from the inlet side end face to the outlet side end face (see, for example, Patent Literature 3).
Patent Literature 1: JP-A-1997-155189
Patent Literature 2: JUM-A-1986-10917
Patent Literature 3: JP-A-1986-4813
However, in the above-mentioned honeycomb structure wherein the section of cell normal to the flow direction of fluid (this section may hereinafter be referred to as “cell section”) has about the same area over the entire flow direction, there is a problem of the high pressure loss caused by the fluid incoming resistance at the inlet side end face of honeycomb structure and the fluid outgoing resistance at the outlet side end face. In order to respond to the recent stricter regulation for exhaust gas, there is a trend of allowing a honeycomb structure to have a higher cell density for increased surface area and, accordingly, there is a tendency of an increase in pressure loss at the inlet side end face and outlet side end face of honeycomb structure. This occurs because, when a fluid (e.g. an exhaust gas) enters the cells, a fluid-stagnant portion appears at the cell inlet side end face; the substantial area of cell section through which the fluid can pass decreases sharply, and flow rate changes in this portion causing a fluid loss. Also at the cell outlet side end face, the area of cell section through which the fluid such as exhaust gas can pass increases sharply and flow rate changes causing a fluid loss. Also in the diesel particulate filter (DPF) used for purification of exhaust gas emitted from diesel engine or the like, there is a problem of the high pressure loss caused by the exhaust gas incoming resistance at the inlet side end face of honeycomb structure and the exhaust gas outgoing resistance at the outlet side end face.
In the diesel particulate filter, there is also the following problem. That is, the captured particulate matter deposits gradually in the filter with its use; the particulate matter adheres to the plugged ends of cells at the inlet side end face of filter; this invites further gradual deposition of particulate matter which leads to plugging of open cell ends at the inlet side end face of filter; as a result, a sharp increase in pressure loss takes place. One reason for this is considered to be that, since, as shown in FIG. 29(a) and FIG. 29(b), there is stagnation of exhaust gas flow at the inlet side end face 64 of each cell 63, particulate matter deposits easily at the inlet side end face 64 of each cell 63. Incidentally, numeral 66 indicates a plugging member which plugs an open end of a given cell 63.
When, as shown in FIG. 30(a) and FIG. 30(b), the outer surface of ceramic-based honeycomb structure 61 is covered with a heat-resistant holding member 68 showing an elasticity when compressed and the resulting honeycomb structure is housed in a container 67 (e.g. a metal-made container) while applying a compression pressure to the honeycomb structure via the holding member 68, to use it as a converter 60, the honeycomb structure 61 has been held in the container 67 by providing a retainer ring 69 at the peripheral area of end face of honeycomb structure 61 via a heat-resistant cushioning member, in order to prevent the occurrence of positional shift of honeycomb structure caused by the pressure of exhaust gas or by the vibration of engine. Ordinarily, a peripheral area of each end face of honeycomb structure 61, having a width of about 5 mm is held by the retainer ring 69. As a result, the open ends of the cells 63a at each peripheral area are blocked by the retainer ring 69, making the passage of exhaust gas difficult, which has invited a decrease in the substantial total area of cell sections of honeycomb structure 61 and an increase in pressure loss. Also in the filter, there is a decrease in the filtration area of cells 63 having the open ends blocked and this has invited a further increase in pressure loss. Further, even when the retainer ring 69 is provided only at the outlet side end face of honeycomb structure 61, the passage of fluid (e.g. exhaust gas) is difficult and an increase in pressure loss is invited when the open ends of cells 63a at the outlet side end face are blocked.
Also, when such a converter 60 is under actual load, there has been a problem that circumferential cracks tend to appear at the peripheral portion of honeycomb structure 61. The position of appearance of circumferential cracks is the middle portion of honeycomb structure 61 in its axial direction or the vicinity of the end of holding member 68. As the axial direction length of honeycomb structure 61 is larger, the circumferential cracks appear more at the middle portion in the axial direction. In a honeycomb structure 61 (honeycomb filter) in which the open ends of given cells 63 are plugged, the circumferential cracks appear more at the vicinity of the end of holding member 68.
When, as described above, the peripheral area of the end face of honeycomb structure 61 is blocked by the retainer ring 69, an exhaust gas is difficult to flow through the cells 63a of the peripheral portion of honeycomb structure 61 as shown in FIG. 31; the portion Y (which is an outer portion of honeycomb structure 61) has relatively low temperatures and the portion X (which is an inner portion constituted by cells 63b not blocked by the retainer ring 69) has relatively high temperatures; accordingly, a temperature difference is generated. Therefore, it is considered that, even when the inner portion X is heated and gives rise to thermal expansion in the axial direction, the outer portion has low temperatures and is unable to follow the inner portion; there appears a tensile stress of axial direction at the outer surface of honeycomb structure 61 and circumferential cracks are generated. Since a pressure is applied to the outer surface by the holding member 68 and the free thermal expansion of the outer surface is restricted, the tensile stress at the outer surface is larger as the pressure by the holding member 68 is larger. This is also true even when part of the cell open ends of honeycomb structure 61 is plugged for use as a filter.
As shown in FIG. 32, when a plugging member 66 is provided at part of the cell 63 open ends of honeycomb structure 61 to use it as a filter, there is discontinuous rigidity at the boundary portion Z, on the cell 63 inner side end face of plugging member 66, between the plugging member 66 and the adjacent open cell 63 not provided with the plugging member 66. Therefore, it is considered that, when the honeycomb structure 61 receives a pressure at the outer surface by the holding member 68 (see FIG. 31) or the like, stress concentration is generated at the outer surface of honeycomb structure 61 in the vicinity of the cell 63 inner side end face of plugging member 66, which generates circumferential cracks. Since a larger pressure may be applied to the end of holding member 68 (see FIG. 31) than to the inner portion of holding member 68 owing to an edge stress action, stress concentration is higher at the outer surface of honeycomb structure 61 in the vicinity of the cell 63 inner side end face of plugging member 66.
Further, when the honeycomb structure is used as a filter for exhaust gas, there has been a problem that cracks are generated at the vicinity of the exhaust gas inlet side end face of honeycomb structure owing to the thermal shock caused by the sharp temperature change of exhaust gas. Cracks are generated in a large amount particularly when the honeycomb structure is mounted in the vicinity of engine where the exhaust gas temperature is relatively high and the temperature change or flow rate change of exhaust gas is very sharp.
Although not shown by drawing, conventional honeycomb structures also have a problem that, when the solid foreign matter such as iron oxide peeled from an exhaust pipe is carried by an exhaust gas and arrives at the honeycomb structure, the inlet side end face of honeycomb structure tends to cause erosion. It has been confirmed that this erosion takes place easily particularly when the honeycomb structure is mounted near an engine and also when the honeycomb structure has thin partition walls or is made of a material of high porosity or large pore diameter.
Also, in a honeycomb structure such as shown in the Patent Literature 3, wherein cell section size is gradually changed over the entire length of honeycomb structure from the inlet side end face to the outlet side end face, there are a problem that cracks appear easily in firing of formed body for production of honeycomb structure and also a problem that it is unable to employ the conventional canning technique using a holding member 68, such as shown in FIG. 30(a) and FIG. 30(b).
The present invention has been made in view of the above-mentioned situation of prior art. The present invention provides a honeycomb structure which can show a low pressure loss as compared with conventional honeycomb structures and which, when used as a filter, can avoid the sharp increase in pressure loss, taking place at the inlet side end face owing to the plugging of cell open ends by particulate matter, can avoid the generation of circumferential cracks, and can avoid the thermal shock and erosion at the end face of honeycomb structure; and a method for manufacturing such a honeycomb structure.